Heretofore, copolymers of polyester and elastomer have generally been made by polymerizing ester-forming monomers including mixtures of elastomeric prepolymers with ester-forming monomers, the result being the formation of a random polyester-elastomer copolymer containing elastomer segments therein. The structures of such copolymers are generally difficult to control and can be the result of side reactions, such as branching.